A novel germ cell determinant reveals parallel pathways for germ line development inCaenorhabditis elegans

Despite the central importance of germ cells for transmission of genetic material, our understanding of the molecular programs that control primordial germ cell (PGC) specification and differentiation are limited. Here, we present findings that X nondisjunction factor-1 (xnd-1), known for its role in regulating meiotic crossover formation, is an early determinant of germ cell fates in Caenorhabditis elegans. xnd-1 mutant embryos display a novel “one PGC” phenotype due to G2 cell cycle arrest of the P4 blastomere. Larvae and adults display smaller germ lines and reduced brood sized consistent with a role for XND-1 in germ cell proliferation. Maternal XND-1 proteins are found in P4 and exclusively localized to the nucleus in the PGCs, Z2 and Z3. Zygotic XND-1 turns on shortly thereafter, at the ∼300-cell stage, making XND-1 the earliest zygotically-expressed gene in the worm PGCs. Strikingly, a subset of xnd-1 mutants lack germ cells, a phenotype shared with nos-2, a member of the conserved Nanos family of germline determinants. We generated a nos-2 null allele and show that nos-2; xnd-1 double mutants display synthetic sterility. Further removal of nos-1 leads to almost complete sterility, with the vast majority of animals without germ cells. The sterility in xnd-1 mutants is correlated with an increase in the transcriptional activation-associated histone modification and aberrant expression of somatic transgenes. Together, these data strongly suggest that xnd-1 defines a new branch for PGC development that functions redundantly with nos-2 and nos-1 to promote germline fates by maintaining transcriptional quiescence and regulating germ cell proliferation.